1. Field of the Invention
The present invention relates to an illumination apparatus and an image reading apparatus, and more particularly, to an image reading apparatus for reading an image by a sequential linear scanning method by illuminating a surface of an original, such as an image scanner, copying machine, and a facsimile.
2. Description of the Related Art
There is a demand for smaller size (particularly, lower-profile), lower cost, and higher speed in an image reading apparatus. On this basis, downsizing of a light receiving sensor has been advanced, and in a reduction optical system, reduction magnification has been further decreased. With this change, a sensor surface experiences insufficient illuminance, and in order to obtain the same image quality as that of the conventional technology, there is a need for a brighter original illumination apparatus. In view of this situation, a line-shaped illumination using a light emitting diode (LED) has been developed along with recent improvement of light emission efficiency of the LED.
In the original illumination apparatus, when placing and reading an original that has an uneven surface due to cut and paste on the original surface to be read, a shade is generated at the uneven portion, causing a problem that the reading performance on the original surface is degraded. To cope with this problem, Japanese Patent No. 4490805 and Japanese Patent Application Laid-Open No. 2010-219600 have disclosed an illumination apparatus that reduces the shade of the uneven portion by illuminating from both sides with respect to a reading optical axis by using a line-shaped illumination employing the LED.
The illumination apparatus disclosed in Japanese Patent No. 4490805 and Japanese Patent Application Laid-Open No. 2010-219600 includes a light guide and a reflecting member, and illuminates a surface to be illuminated from a first side and a second side with respect to a normal at the center of the surface to be illuminated. The light guide includes an incident surface into which light from a light source enters, a first exit surface from which a portion of the incident light exits via the reflecting surface, and a second exit surface from which a portion of the incident light exits directly by passing through the inside.
In order to achieve high speed reading, it is required to have illumination efficiency with which a sufficient light amount is obtained on the original surface by condensing the light in a sub-scanning direction (when an array direction of the light source is a main scanning direction, the sub-scanning direction is a direction orthogonal to the array direction). However, in the illumination apparatus disclosed in Japanese Patent No. 4490805 and Japanese Patent Application Laid-Open No. 2010-219600, the first exit surface and the second exit surface have no condensing action for causing a converging light beam to exit in a plane orthogonal to the array direction of the light source. For this reason, the illumination efficiency is poor, and it is hard to obtain a sufficient light amount on the original surface.
In the illumination apparatus disclosed in Japanese Patent No. 4490805, the incident surface, rather than the exit surface, is formed in a convex shape on the light source side to have a condensing action. In this case, only light within a small angle range of the incident light beam enters the light guide, and other surrounding light does not enter the light guide, resulting in poor illumination efficiency. In order to solve this problem, it is required to increase the size of the incident surface, which is an obstacle in achieving the smaller size, particularly the lower-profile.